Breaking Dawn

Mary makes her first appearance in Breaking Dawn, where she helps the Cullens as a witness for Renesmee against the Volturi in their confrontation against the Cullens. Emmett and Rosalie tracked her down and sent her to Forks without giving much details other than their family needing her help.

While on her way to Forks, she befriends a nomadic vampire named Randall, who is also on her way to the Cullens. As soon as he heard their story about Bella and Edward's daughter, Renesmee, being mistaken for an immortal child, she agrees to stay and witness for the Cullens.

When Alistair left out of fear of the Volturi, she bravely declares her intent to remain with the Cullens.

Physical appearance

Other than her dark red eyes, Mary's physical description was never explored in the books. In the movie, she is dark-skinned and has thick brown curls.

Personality

Mary's characteristics are not at all described in the books.

In the movie, she is described to be the type of person who comes of her own volition, and never returns to the same place unless she is needed, and is truly a nomadic person since she has no prior relationship and wanders all alone in the world. However, because of this, the bond of family is foreign to her. But despite this, "she is one of their strongest friends so ended up valuable".